A fuel cell power system is an apparatus that electrochemically converts the chemical energy of a fuel, such as hydrogen, into an electrical energy by supplying the fuel and an oxidant, such as air, to a fuel cell proper to react and then extracts the electrical energy. The fuel cell power system is relatively small but functions efficiently, and operates environmentally friendly. In addition, it can be used as a cogeneration system by recovering heat generated by power generation to produce hot water or steam.
Such fuel cells are classified into various types depending on the type of electrolyte. Among these fuel cells, solid polymer fuel cells using a solid polymer electrolyte membrane as the electrolyte can operate at low temperatures and exhibit high power density, and is accordingly suitable for use as a small cogeneration system provided with an eye to general household use or as a motive power source of electric vehicles. It is thus expected that the market scale of solid polymer fuel cells will expand rapidly.
The solid polymer fuel cell power system, for example, a small cogeneration system for general household use, includes: a reformer that produces a hydrogen-containing gas from a hydrocarbon fuel represented by town gas or LPG; a fuel cell stack that generates an electromotive force by supplying the hydrogen-containing gas produced by the reformer and ambient air to a anode and an cathode respectively; an electrical controller that supplies electric energy generated in the fuel cell stack to an external load; and a thermal recycling system that recovers heat generated through the power generation.
A fuel is thus supplied to the fuel cell power system so as to operate it, and as the power generation efficiency, which is defined by a percentage of output to the amount of fuel supplied, is increased, fuel consumption can be reduced and user advantages can be increased. Hence, the power generation efficiency is an index of the performance of the fuel cell power system.
The fuel cell stack, which actually functions to generate power in the fuel cell power system, decreases in voltage with time due to various factors associated with the operation, consequently reducing the power generation efficiency of the fuel cell power system. In order to achieve a fuel cell power system exhibiting a high power generation, efficiency, it is most important that the voltage of the fuel cell stack is prevented from decreasing with time.
In general, the operation of the fuel cell power system is suspended in regular intervals according to the power demand of the user. While the suspended state is retained with the reaction gas supply stopped, air enters the anode and cathode of the fuel cell from the outside. If a hydrogen-rich gas is supplied to the anode at start-up with oxygen present at both the anode and the cathode, the catalyst of the cathode is locally degraded to reduce the voltage of the fuel cell stack. It is therefore necessary to reduce at least the partial pressure of oxygen in the cathode in advance.
In order to reduce the oxygen partial pressure in the cathode, some methods have been known. For example, the oxygen may be purged with nitrogen before starting the operation of the fuel cell power system, or when a fuel is supplied, a fixed load may be connected so that oxygen remaining in the cathode is consumed. If the cathode is allowed to stand in an oxygen atmosphere while the suspended state of the fuel cell system is retained, the catalyst of the cathode is sintered by the cathode kept high potential, or the electrolyte membrane is degraded by oxygen permeating the oxygen anode. Therefore, the cathode is preferably held in a reducing atmosphere even while the suspended state is retained.
For suspending power generation, in a widely employed practice, the oxygen partial pressure of the cathode is reduced by, for example, nitrogen purge, and then the fuel cell is sealed (see, for example, Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-6166). Alternatively, water is electrolyzed with the fuel cell connected to an external power supply, and the cathode is filled with hydrogen-containing gas. Then, the fuel cell is sealed (see, for example, Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-93448). This technique allows complete removal of residual oxygen from the cathode, which is difficult for the technique using nitrogen purge.
However, the methods for retaining the suspended state of the known fuel cell power system provide the following disadvantages.
In the method described in the Patent Document 1, a small amount of air gradually enters the fuel ceil from the outside while the suspended state is retained because of the aged degradation and functional limit of the sealant. As a result, the time for which the fuel cell is held in a reducing atmosphere is reduced. Therefore, the method of the Patent Document 1 is not suitable for long-time suspension.
On the other hand, the method described in the Patent Document 2 overcomes the disadvantage of the method described in the Patent Document 1. However, in order to electrolyze water to generate hydrogen in the cathode, the voltage applied to the anode of the fuel cell needs to be set at least the equilibrium potential 1.22 V under standard conditions. Since the anode is thus kept at a high potential, the catalyst is sintered or its carrier carbon is corroded, degrading the catalyst undesirably.
If an alloy catalyst having high CO resistance, such as PtRu, is used as the catalyst of the anode, elution of constituents of the alloy, such as Ru, is accelerated by the increase of the potential of the anode, thereby reducing the CO resistance.
Solid polymer fuel cells require that the electrolyte membrane are wetted with a predetermined amount of water and hold the water. If the water is electrolyzed while the suspended state is retained, the water content in the electrolyte membrane is reduced accompanying the consumption of the water, so that the performance of the fuel cell is degraded after restarting the operation.
The present invention was conceived to overcome the above-described disadvantages, and an object of the invention is to provide a fuel cell power system that can prevent the degradation of the catalyst by suppressing the increase of the oxygen partial pressure in the fuel cell stack over a long time and thus can prevent the degradation of the performance of the fuel cell caused when the fuel cell power system is brought into the power generation-suspended state or retains the suspended state, and also to provide a method and a program for retaining the suspended state.